The present invention relates to monitoring systems for determining the efficacy of sterilization cycles in sterilizers and, more especially, to a sterilant challenge device for use in such a system.
One of the factors that has a detrimental effect on the efficacy of a sterilization cycle is the presence, in the sterilization chamber, of air or non-condensable gases which can collect in pockets in the load that is being sterilized and prevent the load from being adequately exposed to sterilant. In a steam sterilizer, for example, the object of the sterilization process is to bring steam of a suitable quality and at an appropriate temperature into contact with all surfaces of the load. Any air/gas pockets that collect within the load create a barrier to steam penetration, particularly when the load is made up of porous materials such as hospital linens or fabrics, and prevent effective sterilization from taking place.
Sterilization monitoring systems and methods which make use of a simulated porous load are known. The simulated load is located in the sterilization chamber and the extent to which sterilant penetrates the load during a sterilization cycle is monitored and used to determine whether or not the cycle was effective.
One form of challenge device is described in EP A 0 419 282. That device includes a container having top and bottom walls with a porous packing material disposed within the container. The packing material challenges the penetration of the sterilant by providing a restricted pathway which acts to impede the flow of the sterilant through the device. A removable lid seals the bottom end of the container, while a hole in the top wall of the container allows for the downward ingress of steam into the packing material within the container. The device includes a chemical indicator for detecting sterilant penetration. If sterilant successfully penetrates the packing material, the chemical indicator sheet will undergo a complete colour change. If the sterilant does not sufficiently penetrate the packing material, the chemical indicator will not undergo a complete uniform colour change, thereby indicating inadequate air removal or the presence of non-condensable gas.
Other challenge devices for use in steam or gas sterilizers are described in EP-A-0 421 760; U.S. Pat. No. 5,066,464; WO 93/21964 and U.S. Pat. No. 5,270,217. In each of those devices, sterilant from the sterilization chamber must cross some form of physical barrier before it reaches a sterilant sensor within the test pack. WO 93/21964, for example, describes a test unit comprising a test cavity having an opening at one end to permit entrance of ambient gases, a temperature sensor at the other end and a heat sink (for example gauze, felt, open-celled polymer foam) between the temperature sensor and the opening.
U.S. Pat. No. 4,594,223 describes various devices for indicating the presence of non-condensable gas in a sterilization chamber. In one version, a heat and humidity sensor is located at the lower end of an elongate cavity which is open at the upper end. Heat sink material in the form of fibrous insulating material is located within the cavity between the opening and the sensor. In another version, the path between the opening and the sensor is through a heat sink block in the form of a mass of aluminium surrounded by insulation, rather than through fibrous heat sink material.
U.S. Pat. No. 4,115,068 describes an air indicating device for use in sterilizers, comprising an upright tube which is open at its bottom end and closed at its top end. The tube is made of heat insulating material lined on its interior surface with a heat conducting material. A thermal indicator strip extends axially into the tube.
Another known arrangement for challenging the penetration of sterilant to a particular location comprises a very long (typically, 1.5 m) stainless steel tube with a narrow bore (typically 2.0 mm) which provides the only access for sterilant to the predetermined location.
WO 97/12637 describes a challenge device which comprises a tube of thermally-insulating material, the bore of which is closed at one end but open at the other for the entry of sterilant, and a plurality of thermally-conductive masses located around the tube and thermally-separated from one another lengthwise of the tube by air gaps. When the device is in use in a sterilization chamber, the penetration of sterilant along the bore of the tube is inhibited by the accumulation of air and/or non-condensable gases within the bore resulting from condensation on the walls of the latter. By measuring the temperature in one of the thermally-conductive masses located towards the closed end of the tube, the presence or absence of sterilant in the adjacent region of the bore of the tube can be detected enabling the efficacy of a sterilization cycle to be determined.
The present invention is concerned with the provision of an improved challenge device for a sterilization monitoring system, which is of comparatively simple construction but which will function reliably to enable ineffective sterilization cycles to be detected.
The present invention provides a sterilant challenge device for use in a sterilizer for determining the efficacy of the air removal stage of a sterilization cycle, the device comprising a tube of thermally-insulating material, the bore of the tube defining a free space which is open at one end for the entry of sterilant and is closed at the other end; and a heat sink portion which surrounds the tube and, when the device is in use in a sterilizer, receives heat preferentially from the bore of the tube whereby the penetration of sterilant along the bore of the tube during a sterilization cycle is inhibited through the accumulation of air and/or non-condensable gas within the free space resulting from condensation on the wall of the bore; the device also comprising means for mounting a sensor to detect the presence of sterilant at, or adjacent, the closed end of the tube; wherein the wall of the bore is provided with a surface structure which, during a sterilization cycle, directs condensate that forms on the wall towards the open end of the bore.
Preferably, the heat sink portion comprises a plurality of thermally-conductive masses located around the tube along the length of the latter; the masses being thermally-separated from one another lengthwise of the tube and being surrounded by thermal insulation.